This invention relates to the field of phototypesetters, and more particularly to flash tube intensity control systems employed therein.
In our previously filed patent application entitled "Digital Flash Intensity Control System for Phototypesetter," U.S. Ser. No. 628,692, filed Nov. 3, 1975, and assigned to the same assignee of the present invention, we described a system for storing letter size codes for controlling the flash intensity for particular letter sizes to be set. Where a particular letter size is to be projected through the optical system, an appropriate code is read out of storage and addresses digitally controlled flash circuits, which convert the codes to an analog signal, which in turn controls the degree of energization and hence the light intensity of the flash tube.
While the above-mentioned system is generally satisfactory, it is deemed desirable to provide a self-testing closed loop system, which automatically produces the correct digital code for any particular character size just before phototypesetting. With this system, the machine is self-testing and only a reference voltage need be established to automatically produce the proper intensity codes for any character size. Thus, the permanent letter size codes for controlling flash intensity for each letter size need not be predetermined and placed into permanent storage upon the manufacture of the phototypesetter and thus, manufacturing costs are reduced. Futhermore, when a second photosensitive medium replaces a first photosensitive medium in the machine, the speed or sensitivity of the second medium may differ from the sensitivity of the first medium, so that it is desirable to produce letter size codes for each particular character size having somewhat varying values to compensate for the change in sensitivity, thereby to maintain the desired image density constant. Also, since the system functions in a closed loop mode, any parameter changes in components for generating and projecting the character images will be automatically compensated for, in contrast with permanent storage of letter size codes for controlling intensities.